Turning a folder of images into one document
The situation is familiar: a set of photographs or screenshots that belong together, and a recipient who wants one file. Receipts for an expense claim, screenshots documenting a bug, photographs of damage for an insurance claim, pages of a document captured one at a time. Sending twelve images means the recipient opens twelve files and works out the order themselves.
Assembling them into a PDF fixes the ordering problem permanently. The sequence you set is the sequence they read, the whole thing is one attachment, and it prints as a document rather than as a pile of pictures.
It also makes the set durable. Individual image files get separated, renamed and lost as they move between systems; a single PDF stays intact, and the relationship between the images stays obvious years later when somebody has to look at it again.
Order, formats and mixed sources
Set the order before you convert rather than relying on filenames. Camera filenames sort by capture time, which is usually right for a sequence of photographs and usually wrong for anything you assembled deliberately. Screenshots are worse, because many systems name them by timestamp to the second and the order looks arbitrary.
Mixed formats are fine — JPEG photographs, PNG screenshots and WebP exports can all sit in the same document. What varies is how they compress: photographs shrink well, screenshots of text do not, and a document mixing both will be dominated in size by whichever type there is more of.
Mixed orientations are also preserved rather than normalised, so a set containing both portrait and landscape shots produces a document with both. If consistency matters more than fidelity, rotate or crop the odd ones out before assembling rather than trying to fix the finished PDF.
Keeping the result a reasonable size
This is where image-to-PDF documents most often go wrong. Twelve photographs straight from a modern phone can produce a PDF of forty megabytes or more, which will bounce off most mail servers and every government upload form.
The fix is to compress once, after assembly, rather than reducing each image beforehand. Compressing the finished document gives a better result for the same target size, because the quality cost is paid once rather than per image, and it is far less work.
If the document has a hard limit to meet, it is worth checking legibility rather than just the file size. A set of receipts compressed until the amounts are unreadable technically satisfies the form and fails the purpose, and whoever receives it will simply ask you to send them again.
When a scan would serve you better than a photograph
Phone cameras have become good enough that photographing documents is the default, and for most purposes it is fine. Where it falls down is anything that will be read closely or processed automatically: small print, dense tables, forms that will be run through OCR, and documents where a shadow across the page obscures a figure.
The practical difference is evenness. A scanner lights the page uniformly and holds it flat; a camera does neither, which is why photographed documents show gradients across the page and slight curvature near the spine of anything bound.
If a scanner is not available, most phones now include a document mode that flattens perspective and corrects lighting, and the results are markedly better than a plain photograph. It costs nothing to use and removes the most common reason a submitted document gets sent back.